The assumption is that most utf8 strings allocated are actually ascii
and that if they are not we will encounter a non-ascii char pretty
quickly.

Review URL: http://codereview.chromium.org/6072004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6099 ce2b1a6d-e550-0410-aec6-3dcde31c8c00